Organic light emitting diodes exhibit a series of advantages which make them attractive for use in optoelectronics. These include the availability of many emission colors, low threshold voltages, fast switchability, small thickness and the possibility of using flexible substrates. Typical areas of application of OLEDs are pixelated display instruments and large-area elements for illumination purposes.
One important development goal for OLEDs is to increase the luminous efficiency and, associated with this, to reduce the power consumption. This is of importance particularly for portable electronic devices, in which only limited energy resources are available.
The majority of the previous developments for maximizing the luminous efficiency have been concerned with increasing the internal quantum efficiency. The latter is defined as the ratio between the photons generated in the diode to the injected electrons. New materials with improved luminescence properties, optimized layer sequences or better adapted electrode materials have contributed to increasing the internal quantum efficiency in recent years.
A different approach for increasing the luminous efficiency consists in improving the extraction efficiency. Extraction efficiency is understood as the probability with which a photon generated in the emission zone can be coupled out of the diode and thus ejected. Coupling-out losses arise as a result of absorption or as a result of waveguidance in one of the layers. Waveguidance is caused by total reflection at the interface of two layers with different refractive indexes. In the case of planar interfaces, the angle between incident and reflected beam does not change apart from the sign. Accordingly, once a photon has been subjected to total reflection, it remains confined in the corresponding layer and cannot be extracted.
The representation of FIG. 1 illustrates the process of waveguidance in the layers of an organic light emitting diode:
A transparent electrode 2 (generally indium tin oxide, ITO for short) is situated on a substrate 3. At least one organic layer 1 is deposited on said electrode, said layer being followed by an electrode 5 (e.g. a cathode). Photons are generated by emitters 6 in the organic layer 1. Only photons which do not remain in one of the layers 1–3 and 5 as a result of waveguidance are extracted. The line IV shows the path of an extracted photon by way of example.
Depending on the layer thickness and refractive index of the individual layers, wave-guiding effects, indicated by the lines I, II and III in FIG. 1, may occur in the organic layer, in the transparent electrode and in the substrate, respectively. Since the layer thicknesses of the organic layers and of the transparent electrode are in the region of the light wavelength or less than that, discrete optical modes I, II, corresponding respectively to lines I, II mentioned above, form in these layers, as a single stationary optical wave with a distinct wavelength. A continuum of modes III, corresponding to line III mentioned above, is present in the substrate as a plurality of optical waves with different wavelengths, and traditional geometric optics can be used to calculate and predict the behavior of the continuum of modes. Therefore, a distinction is made between layer modes and substrate modes.
If the formation of layer modes can be disturbed and the undesirable waveguidance can thus be minimized, the extraction efficiency is increased and so, too, is the luminous efficiency.
The optical modes formed in a thin planar layer are dependent not only on the wavelength but essentially on the layer thickness and the refractive index of the layer.
A change in the refractive index and thus the disturbance of the wave-guiding effects can be achieved by means of inhomogeneities in the organic layer if the inhomogeneities have a different refractive index than the layer matrix.
To date, the approaches a) to c) briefly outlined below have been proposed for suppressing wave-guiding effects, in which case a) and b) aim at changing the refractive index and c) utilizes a change in the layer thickness:
a) Dispersion of nanoparticles in one of the organic layers of the OLED (S. A. Carter et al., Enhanced luminance in polymer composite light emitting devices, Appl. Phys. Lett. 71(9), p. 1145, 1997)
In this case, particles of TiO2, SiO2 or Al2O3 having a size of 30 to 80 nm are embedded in the polymeric emitter material MEH-PPV.
This method is associated with the following difficulties:
It is technically very difficult to disperse nanoparticles uniformly in a solvent in which polymers have already been dispersed or dissolved. The consequence of poorly dispersed nanoparticles is an inhomogeneous emission of the LED layer which contains said nanoparticles, for which reason diodes produced in such a way are not suitable for use in display instruments.
The oxidic nanoparticles proposed can lead to a degradation of the active layer through oxidation. Accordingly, a significantly shorter service life has been observed in the case of the diodes with nanoparticles than in the case of the reference diodes.
b) Densely packed SiO2 microspheres (T. Yamasaki, K. Sumioka, T. Tsutsui: Organic light-emitting device with an ordered monolayer of silica microspheres as a scattering medium, Appl. Phys. Lett. 76(10), p. 1243, 2000).
In this case, monolayers of densely packed SiO2 spheres with a diameter of 550 nm are used as scattering centers. In this case, the spheres are applied on the substrate beside the ITO anode tracks. This makes it possible to suppress waveguidance in the organic layers and in the glass. An increase in the coupling-out efficiency was observed.
This method is associated with the following difficulties:
The application of densely packed sphere areas can be realized only with a very high effort. Moreover, a large-area application of such areas of several square centimeters has not yet been realized hitherto. The scattering centers are situated outside the active diode volume. Accordingly, only a small part of the substrate surface can be utilized for light generation. Furthermore, an inhomogeneous luminance results.
The periodic structure of the dense sphere packing means that the scattering efficiency is highly wavelength-selective. Accordingly, a generally undesirable lateral color profile occurs.
c) Corrugated (wave-shaped) organic layers (J. M. Lupton et al., Bragg scattering from periodically microstructured light emitting diodes, Appl. Phys. Lett. 77(21), p. 3340, 2000).
In this case, a polymeric LED is applied on a one-dimensionally periodic structure with a period of 388 nm and depths of 10–100 nm. The structure acts as a Bragg reflector and again leads to scattering of optical modes in the emitter material.
This method is associated with the following difficulties:
The periodicity of the structure leads to a high degree of angular dispersion. A 15 nm thin gold layer was used as the anode, said gold layer already having a high degree of absorption despite the small layer thickness. Transferring the corrugation to the transparent ITO that is otherwise customary as a standard anode is difficult to realize on account of the larger ITO layer thicknesses and high process temperatures.
Furthermore, there are diverse approaches for better coupling-out of the substrate modes. However, since these methods cannot prevent the formation of optical modes in thin layers, they are not taken into consideration for preventing the wave-guiding effect.